SCENE OF HORROR: A bystander comes to the aid of Mary Celine Graham yesterday after the nun and two others were mowed down on Lenox Avenue. The "selfless" sister was later pronounced dead at Harlem Hospital. Photo: James Messerschmidt

A humble Harlem nun with more than 60 years of service was killed yesterday just yards from her convent when a fleeing robber’s getaway van jumped a curb and mowed her down along with two other pedestrians, police said.

Sister Mary Celine Graham, 84, was walking along Lenox Avenue near West 122nd Street with a nurse’s aide shortly before 9:30 a.m. when the minivan collided with another vehicle and spun across the wide avenue’s median, striking the nun, the aide and another pedestrian.

Graham, who suffered from Parkinson’s disease and diabetes, was on her way to a physical-therapy session when she was struck, according to several associates.

Police said the tragic chain of events began at the same intersection, when three men robbed a lawyer at gunpoint and fled in a Chrysler Pacifica.

Minutes later, cops spotted the van and stopped it at West 141st Street and Lenox Avenue.

After cops pulled the driver out and placed him under arrest, another thief brazenly slid behind the wheel, made a U-turn and sped south along Lenox Avenue with the third bandit still inside.

After about a mile, it collided with another minivan that was swerving to avoid a street sweeper.

Witnesses said a 15-year-old boy was thrown from the second minivan, and the driver, his mother, was seriously injured.

“She drove past me and went around the street sweeper and got hit,” said Wayne Harris, 42, a barbershop owner who was parking a car at the time of the collision. “The impact was bad. It sounded like an explosion. Bodies were flying.”

Leon Lewis, 53, another witness, said he couldn’t believe his eyes.

“The car was speeding,” Lewis said. “I thought I was seeing a movie. I thought they were filming an action movie. It was crazy.”

Paramedics rushed to the scene, where the mother could be heard yelling, “Where’s my son?”

She and the teen were rushed to St. Luke’s Hospital. The pedestrians — Graham, nurse’s aide Patricia Cruz, 58 and construction worker Steven Phan, 26 — were taken to Harlem Hospital, where Graham died.

Cruz, who suffered liver damage, had cared for Graham for nearly 10 years. Relatives were reluctant to tell her about the nun’s death.

“My mother keeps asking about Sister Mary,” said Cruz’s daughter, Delia Feelal. “And we just tell her to rest. We’re not telling her right now. They told us we should wait.”

Phan, who suffered two broken legs and a cracked skull, was making a coffee run for his co-workers.

The two suspects fled from the crumpled Pacifica and were on the lam. William Robbins, 18, was in custody after the traffic stop on West 141st.

Cops recovered a gun from the minivan. Officials said the vehicle also matched the description of one used in a pair of robberies Monday.

Witnesses said a patrol car was tailing about a block behind the fleeing minivan with flashing lights and a siren that was being used at intervals.

Police insisted the patrol car was not “chasing” the vehicle, but was tailing the suspects.

Graham served with the Franciscan Handmaids of the Most Pure Heart of Mary, a predominantly African-American convent in Harlem, which she joined in 1948. Associates said she spent her time working with preschool children.

“She was a Franciscan Handmaid to her toes,” said Sister Loretta Theresa Richards, the congregation leader, who joined the same day as Graham.

“In her selfless dedication to serving her children, she was always trying to make things better for somebody else.”